Getting a Damsel & Clown to Co-Exist

[MMAX]

I have a 20 gal tank and put in a yellowtail damsel. He was doing fine for a couple of weeks then I added a common clown about 1/2 the size of the damsel. It didn't take very long for the damsel to start beating on the little guy. Since I paid more for the clown, I caged the damsel in a floating box. I'm considering returning the damsel, but if someone has a suggestion on what I could try it would be helpful. I realize a 20 gal tank is small and the damsel probably claimed the whole thing as "his". If I do have to return him, would adding another clown the same size as my first one be ok or would they fight as well?